As will be better understood from the following description, the present invention was developed for use in the reduction of low to mid frequency noise in the interior, e.g., cabin, of a jet aircraft fuselage. However, while developed for use in an aircraft environment, the invention can also be used in other environments. For example, the invention can be used in other reinforced skin structure vehicles, such as train cars, boats, etc., to reduce low to mid frequency noise. Further, the invention could be used in stationary reinforced skin structures (if constructed of appropriate elements) to reduce the amount of low to mid frequency interior noise in such structures. In addition to reducing low to mid frequency interior noise, the invention also reduces sonic fatigue. More specifically, sonic fatigue occurs as a result of the vibration of structural components. As will be understood from the following description of the invention, the invention reduces noise by reducing the vibration of structural components. As a result, the invention also reduces sonic fatigue, or better stated, increases the sonic fatigue life of a reinforced skin structure incorporating the invention.
Noise inside of a reinforced skin structure, such as the cabin of an aircraft, affects speech communication, comfort, composure and sleep. As a result, it is desirable to maintain interior noise as low as possible. When weight and size are insignificant factors, the designer has relatively wide latitude in the choice of materials and techniques that can be utilized to reduce interior noise. However, in many environments, severe constraints are placed on what steps a designer can take to reduce interior noise. For example, the high power and light weight requirements of commercial jet aircraft severely restrict the devices and techniques that an aircraft designer can use to reduce interior noise to a level comparable with that of other forms of transportion.
It has been found that aircraft cabin noise in the 600 Hz and above frequency range can be treated adequately with skin damping tape, lead vinyl sheeting and fiberglass insulation. However, these devices for treating noise are virtually ineffective at low to mid frequencies. As a result, low to mid frequency cabin noise has remained undesirably high. Moreover, recently developed short takeoff and landing (STOL) aircraft, such as upper surface blown (USB) and externally blown flap (EBF) aircraft, have even higher levels of low to mid frequency interior noise than previously developed jet aircraft. Increased low to mid frequency interior noise occurs because of the proximity of the jet engines of STOL aircraft to the fuselage, the production of low to mid frequency noise disturbances created during the operation of such aircraft, and the fuselage excitation caused by the high degree of correlation and coherence of the fluctuating pressure field surrounding the aircraft. As a result of these new jet aircraft developments, it has become even more important that methods and apparatus for reducing low to mid frequency aircraft cabin noise be developed.
In the past, it was generally believed that aircraft cabin noise below about 600 Hz was controlled by the structural stiffness of the aircraft's fuselage. As a result, attempts to reduce low to mid frequency cabin noise were directed toward increasing the structural stiffness of the fuselage. The number of stiff stringers in one modern aircraft was doubled, in an attempt to reduce low to mid frequency cabin noise. However, test data on this aircraft indicated that although a 100 percent increase in the stringer weight was effective in reducing cable noise in the 300-600 Hz band, it was very ineffective below 300 Hz. Thus, although this approach improved the subjective impression of the cabin noise level, the overall sound pressure level was virtually uneffected.
In another attempt to reduce low to mid frequency noise, nomex honeycomb panels were bonded to the fuselage skin panels of a jet aircraft cabin between the stringers and the frames. Although a significant improvement was made in the 600 Hz and above frequency range, the low to mid frequency noise level and the overall sound pressure level remained virtually uneffected.
More recently it has been found that low frequency cabin noise at takeoff (i.e., when the fuselage structure is unpressurized) can be significantly reduced by designing the fuselage such that the skin and stringers are intrinsically tuned and then damping the vibration of the stringer flanges. In this regard, attention is directed to U.S. Pat. No. 3,976,269 entitled "Intrinsically Tuned Structural Panel" by Gautam SenGupta. While the invention described in this patent is useful in designing a fuselage so that low frequency noise at takeoff is low and so that the sonic fatigue life of the stiffened fuselage skin is improved, these beneficial advantages are lessened during cruise because inplane loads due to cabin pressurization play a very significant role in determining the fuselage structural response to external disturbances, e.g., jet noise and boundary layer turbulence. Because the cruise portion constitutes a substantial portion of most fights, it is, of course, desirable that low to mid frequency noise be maintained at as low as level as possible during cruise, as well as during takeoff and landing.
Therefore, it is an object of this invention to provide a method and apparatus for reducing low to mid frequency noise.
It is another object of this invention to provide a method of an and apparatus for reducing low to mid frequency noise in the interior of a reinforced skin structure.
It is a further object of this invention to provide a method of and an apparatus for reducing the low to mid frequency noise in the interior of a structure comprising a skin supported by reinforcing components, and simultaneously extend the fatigue life of the structure.
It is yet another object of this invention to provide a method of and an apparatus for reducing the vibrational response of an aircraft fuselage to low to mid frequency disturbances and, thereby, reduce the low to mid frequency interior noise in the cabin of the aircraft and, increase the sonic fatigue life of the aircraft.
It is still another object of this invention to provide a method and apparatus for reducing the interior noise in the cabin of an aircraft that is functional during all phases of a flight.